There have been no lack of attempts in the past to improve the properties of various polyethylenes by additives and/or by means of chemical reactions.
Thus, EP-A-170,790 describes a composition which, in addition to ultrahigh molecular weight polyethylene, contains from 0.1 to 25% by weight, based on the polyethylene, of an organic carboxylic acid containing at least 2 carboxyl groups and from 0.01 to 2.5% by weight, also based on the polyethylene, of a Lewis base. This composition is obtained by mixing the ultrahigh molecular weight polyethylene with the organic carboxylic acid and the Lewis base, in particular zinc stearate or aluminum stearate. This mixture can subsequently be processed at elevated temperatures, for example with the aid of a screw extruder. This type of processing is possible since the organic carboxylic acid and the Lewis base surround the ultrahigh molecular weight polyethylene particles and act as a lubricant during processing.
U.S. Pat. No. 4,147,740 describes a process for the preparation of modified, grafted polyethylene in which polyethylene is mixed with from 0.1 to 1.5% by weight, based on the polyethylene, of an unsaturated polycarboxylic anhydride, in particular maleic anhydride. The components are reacted at from 140.degree. C. to 210.degree. C. as a melt in the presence of an organic peroxide having a half life of at least 60 seconds. However, there is no indication of the use of ultrahigh molecular weight polyethylene in the preparation of modified, grafted polyethylenes.
Ultrahigh molecular weight polyethylene (hereinafter ultra polyethylene) means a polyethylene having a very high degree of polymerization and prepared under low-pressure synthetic conditions using Ziegler catalysts. Its mean average molecular weight, measured viscosimetrically, is at least 1,000,000 g/mol. A number of technically important properties, such as notched impact strength, heat distortion resistance, tear strength at elevated temperature, and wear resistance improve with increasing molecular weight. This combination of features allows ultra polyethylene to be used in areas where high demands are made with respect to stress and load-bearing capacity not only at high temperatures, but also at low temperatures. However, the lack of adhesive strength of ultra polyethylene proves to be disadvantageous in the surface-coating of shaped products.
In order to use ultra polyethylene as a coating material, the surface of the ultra polyethylene is subjected to separate treatment. Flame treatment, roughening, spark discharge, treatment with solvents, or dipping into a pickling bath provides the surface of the ultra polyethylene with an adhesive capacity which facilitates further processing.
There is thus a demand for a substance which has essentially the features of ultra polyethylene, has improved abrasion resistance and, at the same time, has good adhesive capacity without, as described above, the necessity for separate after-treatment.